Acoustical instrument



March 9 1926.

A. L. 'I'HURAS ACOUSTICAL INSTRUMENT Filed Dec. 20, 1924 Mex/fox? /l/fier/ 7770/45,

Patented Marl 9, 1926.

uurrsn STATES PATENT oFFIcEQ ALBERT L. Tnunas, E EAST onANeE, NEW JERSEY, ASSIGNOR To WESTERN ELEC- Tmc couramr, iNconronaTi-zn, OF NEw YORK,"N. Y., A CORPORATION or NEW Yonx'.

ACOUSTICAL INSTRUMENT.

Application filed December 20, 1924. Serial No. 757,277.

To all whom it may concern:

-Be it known that I, ALBERT L. THUnAs, a citizen of the United States of America, residing at 75 Ampere Parkway,.East Qrange, in the county of Essex, State of New Jersey, have invented certainnew and useful Improvements in Acoustical Instruments, of which the following is a full, clear, concise, and exact description.

This invention relates to acoustical instruments and has for an, object -an imroved sound passage for instruments used 1n the reception or transmission of sound.

According toone form of this invention, the sound passage comprises a stationary wall through which is an opening leading to a sound deliverin'gmeans and a diaphragm movable toward and vaway from the stationmy wall through the medium of suitable means,vsuch, for example, as thestylus of a phonograph -'or the electromagnet of a telephone receiver; The stationary wall of the sound box' is so shaped relative to the diaphragm that the instantaneous vibrational velocity of the air throughout the sound box is constant. With this form of chamber, frictional losses are reducedto a minimum and the maximum quality of sound reproduction is obtained.

Referring now to the drawings, Fig. 1 is a section through .a phonograph sound box embodying the invention and Fig. 2 is a similar view of a modified form.

The sound box shown in Fig. 1 of the ber leads .to a suitable soun delivering means (not shown A, stylus .14 is con-.

- nectedto the dlaphragm through, the

oscillations of medium of a. rod wherebfy h d1 h t e 'ap ragm the stylus cause vibration o The surface 11 is so curved with respect to the diaphragm 12 that the instantaneous vibrational velocity of the air throu bout the sound box is constant. This desire condition is found to prevail when the ratio of the volume of air displaced by an annular element of the diaphragm 12 to the area of the opening through which the air is displaced is the same asthe ratio of the total volume displaced by the diaphragm 12 to the area of the opening 13. Thus, considering the volume of air displaced by the annulus a the' ratio between it-and the area of the opening b between the edge from the annulus and the-face 11 will be the same as the ratio between the volume pf air dlsplaced by the annulus a and the area of the opening 11 between the edge of the annulus a and the face 11, this ratio being 7 equal to the ratio of the total volume of air displaced by the diaphragm 12 to the area of the opening 13.

The shape as follows: Let

3 =f(m) =cross sectional, shape ofthe diaphragm 12 deflected.

-y I (m) =cross sectional shape of the face b=the radius of the o ening 13.

a=the radius of the iaphragm 12. Then for constant instantaneous vibrational velocity throughout the chamber of the face 11 may be obtained In sound producersthe term )(a:) ofthis equation is small or negligible compared to the term 93 Lama, v p 22; J af (2:)da: Integrating this equation for various values of :11 determines the shape of the inner wall 11.

In Fig. 2, the diaphragm 12 is of the plunger type and consequently its amplitude of v1- bration (d) is constant. As a result the rela tionship necessary to produce constant instantaneous vibrational velocity of air throughout the sound box is simplified. In this instance, the desired conditionis obtainedwhere the ratio of the area of the annulus a; to the area of the opening 1),, equals the ratio between the area of a and areas 'of the opening 6, and also equals the ratio of the efbeing 'sha vibrationa fective area of the diaphragmjto the area of the opening 13. this instance, the

equation 7 '2. fihzfw a 2w mime (3) becomes J Hu -Am 4 2; at 9 --whi1 1'an be readily solved by substituting valuesof -az. v I.

I What is claimed is:

1. In an "acoustic apparatus, a sound; "chamber, and means for setting up sound vibrations in said chamber,.-sa1dchamber (1 to cause the instantaneous 2. In an acoustic apparatus a sound chamber; a diaphra m across said chamber for setting up soun a soundlpassage communicating with thesound chamber, the separation between the diglphragm and each point of the. chamber w 1 leading to the sound passage being such that the vibrational velocity of the:

velocity of the air to be sub-' stantially constant throughout the chaJnvibrations therein, and

waves fromall portions of the diaphragm is'substantially the same.

'3. A sound box for, acoustical instruments jcomprisin a stationary wall having a delivery ori co and a diaphragm, the inner face ofthe' stationary wall bemg so shaped that the ratio of the voLume of air displaced by an annulusof the-diaphragm to the area of the opening through which the air is displaced equals the ratio'of the total volume of air d1s'placed by the diaphragm to the area of said delivery orifice.

' 4. A soundbox for acoustical instruments comprislnga stationary 'wall 'having a delivery orifice and a diaphragm, the inner.

face of the stationary wallbei'ng so shaped 1 that the ratio of the area of an annulus of the diaphragm to the area of the surface betweenthe edge of said annulus and inner face of said wall equals the ration-between the'areavof the diaphragm and the area of the delivery orifice.

In witness whereof, I hereunto subscribe my name this 20th 1924.

f v 5 ALBERT. L. THURAS.

day of December D., 

